The present invention relates generally to printheads for business machines, and more particularly to a method of repairing spent or worn printheads and the product thereof, thereby obviating the need to purchase a new printhead.
Printheads of many types are available for use with electronic printers such as those employed in electronic cash registers, automatic teller machines or other business machines. In many cases, the printheads employed in such machines are of the dot-matrix variety. Such printheads effect the dot-matrix symbol or character by selectively firing print pins which are longitudinally arranged within the printhead. It is noted that a printhead may include a printhead subassembly (nose piece), print pins, a roller guide, an armature, coils, springs, etc. An example of such a printhead subassembly is shown in FIG. 1, wherein the print pins can be seen in the void between the nose section and main body, as well as in FIG. 1A, wherein the longitudinal arrangement of the print pins can be seen in the cross-section. The print pins of such printheads are made of tungsten steel or other steel alloys.
Typically, the nose section of the printhead, or at least the print pin guide section thereof (preferably near the print surface), is ceramic, metallic or plastic depending upon the intended application of the printhead, an original manufacturer's specifications, etc. In many cases, a prefabricated guide section, referred to as a ruby guide, is employed in the nose section to facilitate the proper alignment of the print pins. Ruby guides are plate-like structures with guide holes therethrough for alignment with the print pin holes in the nose section. The plate-like structure would be secured in a cavity in the print surface of the nose section. The advantage to using such ruby guides is that they can be prefabricated from stronger and more durable materials than the plastics often used in the rest of many printheads.
As a result of extensive use, the print pin holes of many printheads, and in particular those made of plastics become worn from contact with the moving print pins. Such wear occurs primarily towards the exit end of the pin holes (print surface), i.e., the end towards which the print pins fire to print a symbol; though in many cases, such wear also occurs at the point of entry into the nose section. Thus, the wear could be extant through the entire length of the nose section or only at the print surface and the entry surface. FIG. 2 is a broken away cross-section of a nose section of a printhead, illustrating several print pin holes or shafts which are worn at the print surface of the nose section and at the entry surface which faces the main body in the void. The print pin holes in FIG. 2 are designed to be, for instance, circular in cross-section, as in FIG. 1, so as to align the pins for proper firing. FIG. 2 shows, albeit somewhat exaggerated in degree, that the material in and around each shaft has been worn from the movement of the respective steel print pins. It is instructive to note that a print pin hole could increase from, for instance, its original diameter of about 0.36 millimeters to a worn diameter of about 0.38 millimeters (at its widest point--usually at the entrance and exit thereof).
As a printhead becomes increasingly worn proximal to the print surface, the print pin holes do not provide the guidance required to ensure that the print pins fire from the nose section in the desired direction. For instance, in FIG. 2, the print pin holes or shafts are intended to guide print pins straight out of the print surface, i.e., at 90.degree. thereto. Without such guidance, the print pins may not and most likely will not fire straight, but rather will stray from their intended path of movement. This is due not only to vibration transmitted to the printhead from its respective machine or printer, but also by reason of the forces and internal stresses associated with the firing of the flexible pins, and the sideward movement of the printer carriage across the platen.
Thus, when the print pin holes or shafts are worn in the above-discussed fashion, it is difficult to print the desired symbol or character with clarity. FIG. 2A shows the letter "I" as it might be printed with a worn printhead. Indeed, the symbols being printed with a worn printhead will become progressively more blurred, and eventually become more difficult to read than the letter "I" shown in FIG. 2A. Therefore, the quality of symbols or characters printed by such a printhead would be degraded, and perhaps become illegible altogether. A printhead whose quality is affected by reason of such wear on the print pin holes and shafts is said to have been "spent".
In addition to the nose section becoming worn, thereby debilitating the guidance to have been provided by the print pin holes or shafts, excessive use of a spent printhead could damage the print pins themselves. That is, the pins might fire without proper guidance and may hit an obstruction which causes a print pin to bend or break.
Once a printhead is spent in the manner described above, it must be replaced. Since new printheads from the original or other manufacturer are quite expensive, there is a considerable market for repaired printheads. There currently exist some effective methods of repairing spent printheads which fire straight from the nose section of the printhead, i.e., where all print pins fire parallel to one another from parallel print pin holes or shafts at a 90.degree. angle to the print surface. One such method is the use of a prefabricated and readily available ruby guide which includes the pattern of the print pin holes or shafts. The prefabricated ruby guide is of a particular thickness and includes holes or shafts running through such thickness in the pattern of the print pin holes in the nose section of the printhead. Thus, the material of the nose section of the printhead is removed from the print surface to a depth equal to the thickness of the replacement ruby guide (and in the specific shape of the replacement ruby guide). The ruby guide is then glued in the cavity formed in the nose section, whereby the holes of the replacement ruby guide would be aligned with the print pin holes of the nose section.
However, the use of prefabricated ruby guide is limited for many reasons. For instance, a prefabricated ruby guide may not be available for the printhead to be repaired or for the particular hole pattern of the printhead to be repaired. By way of example only, ruby guides are not available for printheads which include at least some print pin holes or shafts which extend at angles to one another such that at least some print pins disposed therein fire at angles to one another. One such printhead is IBM's Model No. 4683 for use in electronic cash registers. As those skilled in the art readily understand, it would be most difficult to prefabricate a ruby guide which would include a hole pattern to match the angled hole pattern of such a printhead. In this context, it should be kept in mind that the print pin holes are approximately 0.36 millimeters or 0.014 inches in diameter, and to match the specific angles of each hole in a ruby guide would be time consuming, expensive and tedious; and is yet to be accomplished. Thus, it might not be cost effective to manufacture ruby guides with angled holes in the large volume required for repairing such printheads. Moreover, even apart from cost, it would be quite difficult to secure such ruby guides in precise alignment with the print pin holes as would be required to properly repair the printhead.
Even with printheads having parallel print pin holes which guide the firing of print pins at 90.degree. to the print surface of the printhead, prefabricated ruby guides may not be available. Beyond expense, difficulty of manufacture and difficulty in application, another reason a prefabricated ruby guide might not be available for a particular printhead is that such a prefabricated ruby guide would only be good for a particular printhead. Therefore, each particular printhead would require its own ruby guide. Still further, although the original equipment manufacturer may have ruby guides readily available to it for use in constructing new printheads, such ruby guides might not be readily available to those in the business of repairing printheads.
On some printheads, it is also difficult to replace a ruby guide or use a ruby guide after removing material since there would not be enough space for the ruby guide on the print surface of the printhead. That is, there would not be enough material around the replacement ruby guide to secure the same in place on the print surface of the printhead.
In addition, the printhead possesses several structural and operational characteristics which must be present in a repaired printhead as well to ensure proper and accurate printing. For instance, a ball bearing arrangement is operatively associated with the IBM 4683 printhead such that the ball bearing will ride along a guide below the platen surface as the printhead moves across the carriage of the printer. The ball bearing arrangement is positioned at a predetermined distance from the print surface to ensure proper character or symbol development by the printhead. In repairing a spent printhead, care must be taken to preserve the relationship between the ball bearing arrangement and the print surface (or the length of the printhead). Thus, with respect to the use of a ruby guide, a precise amount of material must be removed so that the ruby guide does not change the characteristics of the printhead.
Apart from purchasing a new printhead from the original equipment manufacturer, one of the only options for replacing many printheads is to mold an entirely new nose piece (printhead subassembly) for the printhead. However, this option is undesirable because of the cost of having a mold made for such a printhead subassembly. For example, a mold for the IBM 4683 printhead subassembly, is upwards of $50,000 to $70,000, and in some cases, with no guarantee as to whether the mold will be capable of molding a useable subassembly for the printhead. Moreover, just as with the ruby guides, the mold would be useful only for the particular printhead, and therefore each particular printhead would require its own subassembly mold.
Another option for repairing printheads is to remove the print pins, and replace them with larger print pins. This too is undesirable since it is a temporary measure at best, and may affect the integrity of the symbols printed by the print pins. Moreover, it is estimated that this technique only works to any extent about 30% of the time.
Recently, an attempt was made to repair a printhead without molding an entirely new nose piece. However this attempt permanently changed the character and integrity of the printhead, and more importantly, lasted only a short period of time. To the extent the technique employed in this attempt can be gleaned from the repaired printhead, it appears to have included the grinding of the print surface in the area of the holes in a lengthwise manner. A strip of plastic, which appeared to be considerably less than one millimeter in thickness, is glued on the ground-away area of the holes. The result, however, is that the strip of plastic is not flush with the print surface, but rather extends beyond the print surface. One of the reasons this might occur is that the thickness of the glue can not be controlled, and indeed may be thicker in one area of the strip of plastic than in another. It is not clear whether the strip of plastic included the hole pattern of the spent printhead or whether the holes were punched or drilled through after the strip of plastic was in place. In any event, the strip is relatively thin and flexible, and therefore it is quite probable that the original print pin holes which remain immediately below the strip of plastic may still be enlarged from having been worn by the original print pins. Thus, abraded material or other material can get behind the strip of plastic in the enlarged portions of the original print pin holes, causing a strip of plastic to flex upon the firing of the replacement print pins. It is thus noted that the strip of plastic is not the same as a ruby guide which is thicker and more durable than a thin strip of plastic.
It is also noted that in this technique it appears that the original print pins were used in the repaired printhead. Perhaps most significantly, the character of the printhead itself is changed by this technique. For example, the ball bearing rider which bears against a guide below the platen as the printhead is moved across the carriage must be of a particular distance from the print surface, as noted above. Since the addition of the strip of plastic is above or below the original print surface, the length of the printhead is greater or smaller, respectively, in that area, depending of course upon the amount of original material removed; and therefore the ball bearing arrangement must be re-aligned with respect to the strip of plastic or new length of the printhead. This, of course, adds tedious and time consuming steps to the repair process. Lastly, it is possible that the practicing of this technique could destroy the repairability of the printhead. That is, the repaired printhead may only last a month or two before failing, and it could not be repaired due to the permanent change in the printhead. Rather, a new printhead would have to be purchased after the failure of the repaired printhead.
Accordingly, to replace many printheads, and in particular those having at least some print pins which fire at angles to one another, the only viable option for a permanent replacement would be to purchase a new printhead. This could become extremely expensive for department store chains and other entities which may have thousands of electronic cash registers, all of which may have worn printheads requiring replacement within months of one another.
The above shortcomings make it clear that an improved method for repairing printheads which use print wire, firing at angles to one another or parallel to one another, and the resulting product thereof, are warranted. The present invention provides such a method and product thereof.